Reexamination of instrument change effects in the US Historical Climatology Network

The homogenized U. S. Historical Climatology Network (HCN) data set contains several statistical adjustments. One of the adjustments directly reflects the effect of instrument changes that occurred in the 1980s. About sixty percent of the U. S. HCN stations were adjusted to reflect this instrument change by use of separate constants applied universally to the monthly average maximum and minimum temperatures regardless of month or location. To test this adjustment, this paper reexamines the effect of instrument change in HCN using available observations. Our results indicate that the magnitudes of bias due to the instrument change at individual stations range from less than - 1.0 degrees C to over + 1.0 degrees C and some stations show no statistical discontinuities associated with instrument changes while others show a discontinuity for either maximum or minimum but not for both. Therefore, the universal constants to adjust for instrument change in the HCN are not appropriate.